THE ORGANS OP RESPIRATION. 167 



their number, two always being present, and secondly by their form, 

 they being simple horny tubes unprovided with setae at their end. 

 In Ranatra they are as long as the body, and in Nepa half its length. 

 It seems to be a very general law, that the situation of the spiracles 

 should be at the posterior end of the body, not only in the Diptera, but 

 also in all larvae which live in water and are unprovided with branchiae. 

 With respect to the larvae of the Diptera, those yet investigated have 

 their spiracles in that situation: for example, the flies and (Estridce. 

 The larvae of the water-beetles likewise (for example, Dyticus and 

 Hydrophilus) have their spiracles at the anal end, contiguous to the 

 anus, and have none at their sides, although Sprengel describes and 

 even figures them there *. 



120. 



III. GILLS, or BRANCHIAE. This third description of the organs 

 of respiration is particularly distinguished from both the others by its 

 want of apertures to admit the air into the tracheae. The gills are 

 processes of the epidermis in the form of hair or leaves, in which 

 delicate tracheae ramify in every direction. These vessels imbibe the 

 air mixed up mechanically with the water, and conduct it to the main 

 stems concealed in the body, by means of the branches of which it passes 

 to all the internal organs. Through this arrangement insects pro- 

 vided with gills do not require atmospheric air, they consequently do 

 not rise to the surface of the water, but live constantly in it concealed 

 among water plants. 



The branchiae may be separated into two divisions, by their forms; 

 the one being delicate and slender, resembling hair, while the other 

 is broad, thin, and lamelliform. 



The hair-shaped branchiae seldom appear singly, but generally in 

 approximate fasciculi, which are formed by either the ramifications of 

 one or of several main stems (PI. III. f. 6.), or by filaments radiating 

 from one point (the same, f. 10). The epidermis of these processes is 

 exceedingly delicate, as well as the small silvery tracheae enclosed by 

 it. This kind of branchiae is the most usual and general ; it is found 

 particularly in the larvae and pupae of the gnats. 



The lamellate branchiae are found only in the Dictyotoptera and the 

 Neuroptera, and appear as broad or pointed lanceolate leaves, and are 

 found on each side of each abdominal segment, or only at its end. 



* CoinmcnUu-., {.. ','>!. N... xx. PI. II. f. '20. 



